Multi purpose drilling system and method

ABSTRACT

A method of forming a multi purpose drilling for degassing a coal seam that is to mined, the method including the steps of drilling a primary borehole from a surface into a zone above the coal seam, such that at least a portion of the primary borehole extends substantially parallel to the coal seam, and drilling at least one secondary borehole, wherein the least one secondary borehole extends from the primary borehole into the coal seam.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage Application under 35 U.S.C. §371 ofPCT Application No. PCT/AU2015/050130, filed, March 24, 2015, which isincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a multi purpose drilling system and method. Theinvention relates in particular, although not exclusively, to a methodof forming a multi purpose drilling for degassing a coal seam that is tobe mined.

BACKGROUND OF THE INVENTION

Before coal seams can be mined, they are typically degassed to removecoal seam gases which would otherwise be lost to the atmosphere and/orpose a danger to the mining operation. Additionally, the coal seam istypically degassed during development, production and after mining (goafdegassing). One prior art solution is to initially drill surface to inseam holes to pre drain the coal seam.

At a later stage, as longwall panels are being developed, underground toin seam holes are drilled from a maingate or a tailgate into a longwallpanel to achieve effective drainage of the coal seam gases before andduring production. These holes are typically connected to pipes in themaingate or the tailgate to convey the coal seam gases to the surface.

Prior and during production, vertical goaf gas drainage wells aredrilled from the surface into a position where the wells can drain thegoaf as the longwall is mined, these vertical wells are typically spaced50 to 200 meters apart.

Although the above mentioned prior art solution is effective at drainingcoal seam gases prior to, during development, production and aftermining, it requires a large amount of capital, effort and time,particularly over the life of a mine.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

OBJECT OF THE INVENTION

It is an object of the invention to overcome or at least alleviate oneor more of the above problems and/or provide the consumer with a usefulor commercial choice.

Other preferred objects of the present invention will become apparentfrom the following description.

SUMMARY OF THE INVENTION

In one form, although it need not be the only or indeed the broadestform, the invention resides in a method of forming a multi purposedrilling for degassing a coal seam that is to be mined, the methodincluding the steps of:

drilling a primary borehole from a surface into a zone above the coalseam, such that at least a portion of the primary borehole extendssubstantially parallel to the coal seam; and

drilling at least one secondary borehole;

wherein the at least one secondary borehole extends from the primaryborehole into the coal seam.

Preferably the step of drilling the primary borehole involves drillingthe primary borehole such that the majority of the primary borehole inthe zone above the coal seam extends substantially parallel to the coalseam.

Normally the step of drilling the primary borehole is performed prior tothe step of drilling the at least one secondary borehole. A skilledaddressee will understand that in one embodiment, the at least onesecondary borehole can be drilled before the primary borehole. Forexample a secondary borehole is drilled from the surface through asurface casing and into the coal seam, and a primary borehole is thendrilled through the surface casing such that it extends from thesecondary borehole into the zone above the coal seam (although thesecondary borehole was drilled first, it still extends from the primaryborehole).

Preferably the zone is a primary caving zone. More preferably the zoneis up to 2.5 times the height of the coal seam, above the coal seam. Forexample, if the height of the coal seam is 6 meters, the zone is up to15 meters above the coal seam. In a preferred embodiment, the zone isbetween 5 to 40 meters above the coal seam. Preferably the zone isbetween 5 to 25 meters above the coal seam. More preferably, the zone isbetween 10 to 15 meters above the coal seam.

Preferably at least a portion of the primary borehole extends parallelor at least at a slight angle to horizontal. For example, if a coal seamextends horizontally, at least a portion of the primary borehole mayextend parallel to −5° to the coal seam. A benefit of at least a portionof the primary borehole extending parallel or at a slight angle tohorizontal is that any water or other liquids in that portion of theprimary borehole may be better controlled.

Preferably the primary borehole is not drilled into an upper coal seam(i.e. a coal seam above the coal seam that is to be mined) in adirection substantially parallel to the upper coal seam.

Preferably a portion of the primary borehole is drilled such that it hasa larger diameter than the average diameter of the at least onesecondary borehole. For example, the average diameter of the at leastone secondary borehole may be in the range of approximately 180 mm (˜7″)to 300 mm (˜12″) and the diameter of the larger portion of the primaryborehole may be in the range of approximately 300 mm (˜12″) toapproximately 610 mm (˜24″).

Preferably the primary borehole is drilled such that the portion of theprimary borehole extending substantially parallel to the coal seam islarger in diameter than a portion of the primary borehole adjacent tothe surface. In one embodiment, the step of drilling the primaryborehole involves drilling a pilot hole and reaming at least a portionof the pilot hole. A skilled addressee will understand that either anadvance or retreat reaming method may be used to ream at least a portionof the pilot hole.

In one embodiment, the step of drilling at least one secondary boreholeinvolves drilling one secondary borehole into the coal seam such that amajority of the secondary borehole in the coal seam extendssubstantially parallel to the coal seam. Preferably the method includesdrilling branching holes from the secondary borehole that extend intothe coal seam. A skilled addressee will understand that the branchingholes extending from the secondary borehole into the coal seam can be inthe form of any scatter branching as desired, such as, but not limitedto, fan, parallel and/or herringbone patterns or the like or acombination thereof at predefined intervals.

In another embodiment, the step of drilling at least one secondaryborehole involves drilling a plurality of secondary boreholes extendingfrom the primary borehole into the coal seam. Preferably, the pluralityof secondary boreholes are drilled such that they extend from theportion of the primary borehole that extends substantially parallel tothe coal seam, and into the coal seam. Preferably, the plurality ofsecondary boreholes are drilled at predefined intervals. Preferably, theplurality of secondary boreholes are drilled in a pattern. For example,the plurality of secondary boreholes may be drilled in any scatterbranching as desired, such as, but not limited to, fan, parallel and/orherringbone patterns or the like or a combination thereof at predefinedintervals when viewed from above.

In another embodiment, the step of drilling at least one secondaryborehole involves drilling at least one secondary pilot hole and reamingeach of the at least one secondary pilot hole.

Preferably the method includes the step of providing a casing in theprimary borehole. Preferably the step of providing a casing in theprimary borehole involves providing a casing that is at least partiallyperforated in the portion of the primary borehole that extendssubstantially parallel to the coal seam. Preferably the casing is afibreglass casing. Alternatively, the casing may be a steel or PVCcasing. The casing may be a fire retardant and/or anti static casing. Abenefit of providing a casing is that borehole stability is improved.

Preferably the method includes the step of providing at least one casingin the at least one secondary borehole. Preferably the at least onecasing is at least one fibreglass casing. Alternatively, the at leastone casing may be at least one steel or PVC casing. The at least onecasing may be fire retardant and/or anti static. Preferably the at leastone casing is at least partially perforated.

Preferably the method includes the step of drilling at least one wellfrom the surface, each of which intersects with the primary borehole.Preferably each well is a vertical well. A skilled addressee willunderstand that a vertical well includes a well that is substantiallyvertical. Preferably there are a plurality of wells. More preferably theplurality of wells are spaced from each other. For example, theplurality of wells are spaced from each other by about 1000 meters.Preferably each well is adapted to convey coal seam gas to the surface.

In one embodiment, the step of drilling at least one well from thesurface, each of which intersects with the primary borehole, involvesdrilling the at least one well such that each well also intersects withone of the at least one secondary borehole.

It will be appreciated that the step of drilling at least one well fromthe surface can be carried out before, during or after the primaryborehole and/or the at least one secondary borehole is drilled.

Preferably the method includes the step of determining the location ofthe coal seam. Preferably, the method includes the step of determiningthe coal seam floor. More preferably, the method includes the step ofdetermining the coal seam roof Preferably the method includes the stepof determining the location of the zone above the coal seam into whichthe primary borehole is drilled.

In another form, the invention resides in a multi purpose drillingsystem for degassing a coal seam that is to be mined, the multi purposedrilling system comprising:

a primary borehole extending from a surface into a zone above the coalseam; and

at least one secondary borehole extending from the primary borehole intothe coal seam;

wherein at least a portion of the primary borehole extends substantiallyparallel to the coal seam.

In one embodiment, a plurality of multi purpose drilling systems areused to degas a coal seam.

Further features of the invention will become apparent from thefollowing description.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist in understanding the invention and to enable a person skilledin the art to put the invention into practical effect, preferredembodiments of the invention will be described by way of example onlywith reference to the accompanying drawings, wherein:

FIGS. 1a-1e show schematic views of various stages of a multi purposedrilling system according to an embodiment of the invention;

FIG. 2 shows a schematic plan view of part of the multi purpose drillingsystem of FIGS. 1a -1 e;

FIG. 3 shows a schematic view of a multi purpose drilling systemaccording to an embodiment of the invention;

FIG. 3a shows a schematic plan view of a multi purpose drilling systemaccording to an embodiment of the invention;

FIG. 4 shows a schematic view of two back to back multi purpose drillingsystems;

FIG. 5 shows a schematic view of two facing multi purpose drillingsystems;

FIG. 6 shows a schematic view two multi purpose drilling systemsoriginating from the same location; and

FIG. 7 shows a schematic view of two overlapping multi purpose drillingsystems.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1a -2 show a multi purpose drilling system 30. In particular,FIGS. 1a-1c show a sequence of drillings to create a multi purposedrilling system, FIGS. 1d-1e show the multi purpose drilling system 30as the coal seam 12 is progressively mined and FIG. 2 shows a plan viewof a portion of the multi purpose drilling system 30.

With reference to FIG. 1 a, a primary borehole 32 is drilled from thesurface 10 to above the coal seam 12. The primary borehole 32transitions to being substantially parallel to the coal seam 12 as itextends in a primary caving zone 14, which is located above the coalseam 12.

A perforated casing (not shown) is inserted into the primary borehole 32(typically during the drilling of the multi purpose drilling system 30).

In FIG. 1 b, a number of secondary boreholes 36 are drilled such thatthey extend from the primary borehole 32 into the coal seam 12. Thesecondary boreholes 36 extend downwardly (as seen in FIG. 1b ) andoutwardly (as seen in FIG. 2) from the primary borehole 32.

In use, the plurality of secondary boreholes 36 pre drain the coal seam12 so that longwall panels (22 as seen in FIG. 2) can be developed.During development and production, the plurality of secondary boreholes36 continue to drain the coal seam gases.

In FIG. 1c a vertical well 40 is drilled from the surface 10 such thatit intersects with the primary borehole 32. The vertical well 40 isadapted to convey coal seam gases to the surface 10.

FIGS. 1d and 1e show the coal seam 12 being mined by a shearer 18, theshearer 18 and the hydraulic jacks 20 advance along the coal seam 12,creating a goaf 16. The coal seam gases from the goaf 16 are drained bythe primary borehole 32.

It will be appreciated that the secondary boreholes 36 will no longerdrain coal seam gases once the corresponding portion of the coal seam 12has been mined by the shearer 18.

A benefit of having a perforated casing in the primary borehole 32 isthat the primary borehole 32 remains operational for a longer time inthe goaf 16 (compared to not having a casing in the primary borehole 32)as the primary caving zone 14 collapses into the goaf 16. A benefit ofdraining the goaf 16 using the primary borehole 32 is that not as manyvertical wells 40 are required (compared to the prior art solution ofdrilling vertical wells every 50-200 meters).

FIG. 2 shows a schematic plan view of part of the multi purpose drillingsystem 30 in relation to a developed longwall panel 22, defined by atailgate 24 and a maingate 26.

It will be appreciated that the multi purpose drilling system 30 istypically drilled before the tailgate 24 and maingate 26 are formed andbefore the longwall panels 22 are developed and the coal seam 12 ismined. The location of the maingate 26 and tailgate 24 are typicallypredetermined and the drilling system 30 is drilled such that the endsof the secondary boreholes 36 stop short of the respective maingate 26or tailgate 24. The secondary boreholes 36 typically stop 5-10 m shortof the respective maingate 26 or tailgate 24. A benefit of stoppingshort of the respective maingate 26 or tailgate 24 is that this preventsgas from escaping from the drilling system 30 into the maingate 26 ortailgate 24.

FIG. 3 shows a further embodiment of a multi purpose drilling system130. The multi purpose drilling system 130 has a primary borehole 132that is drilled from the surface 10 to above the coal seam 12. It willbe appreciated that drilling can include reaming and that drilling theprimary borehole 132 may include reaming a pilot hole.

The primary borehole 132 transitions to being substantially parallel tothe coal seam 12. It will be understood that in the presentspecification, substantially parallel to the coal seam 12 includes ±5°to the coal seam 12. The portion of the primary borehole 132 thatextends substantially parallel to the coal seam 12 is preferably 5 to 40meters above the coal seam 12 depending on the strata and thepermeability of the strata above the coal seam.

Similar to the embodiment shown in FIGS. 1A-1E, in the embodiment shownin FIG. 3, a perforated casing (not shown) is inserted into the primaryborehole 132. A benefit of having a perforated casing is that theprimary borehole 132 remains operational for a longer time after thecoal seam 3 has been mined below it.

A secondary borehole 136 extends from the primary borehole 132 into thecoal seam 12. Branching holes 142 extend from the secondary borehole 136into the coal seam 12. A casing (not shown) is inserted into thesecondary borehole 136. A benefit of casing the secondary borehole 136is that the casing maintains borehole stability.

Typically the primary borehole 132 is drilled before the secondaryborehole 136 is drilled. A skilled addressee will understand that in oneembodiment, the secondary borehole 136 can be drilled before the primaryborehole 132. An advantage of drilling the secondary borehole 136 firstis that the coal seam roof can be accurately located (using for examplegamma detection from the secondary borehole 136) such that a portion ofthe primary borehole 132 can be drilled so that it can extendsubstantially parallel to the coal seam 12.

The primary borehole 132 is typically drilled such that the portion ofthe primary borehole 132 that extends substantially parallel to the coalseam 12 has a larger diameter than the secondary borehole 136. Forexample, the average diameter of the secondary borehole 136 isapproximately 205 mm (˜8″) and the larger diameter portion of theprimary borehole 132 has a diameter of approximately 432 mm (˜17″). Thelarger diameter portion of the primary borehole 132 aids in draining thegoaf (not shown) once the coal seam 12 below the primary borehole 132has been mined.

Vertical wells 140 are drilled from the surface 10. The vertical wells140 are spaced approximately 800-1000 meters apart. The vertical wells140 intersect the primary borehole 132 and the secondary borehole 136.The vertical wells 140 assist in conveying the coal seam gases to thesurface 10 as well as draining water and other liquids from the primaryborehole 132 and the secondary borehole 136.

FIG. 3a shows a schematic plan view of part of the multi purposedrilling system 130 in relation to a developed longwall panel 22,defined by a tailgate 24 and a maingate 26. In this embodiment, it canbe seen that the branching holes 142 extend from the secondary borehole136 in a ‘herringbone’ pattern.

FIGS. 4, 5, 6 and 7 show various combinations of multi purpose drillingsystems used to degas a coal seam 12. It will be appreciated that it canbecome increasingly difficult to push casing longer distances. Forexample it may be difficult to push casing further than 1800 meters. Assome longwall panels are longer than 1800 meters, it may be necessary toprovide more than one multi purpose drilling system to degas a coal seam12.

FIG. 4 shows a tandem configuration, where two multi purpose drillingsystems 230, 330 are drilled to degas a portion of the coal seam 12. Theportion of the coal seam 12 to be degassed is a proposed longwall panelthat extends from a main road 160 to an install road 162. The arrow 164indicates the direction of mining, from the install road 162 towards themain road 160.

The first multi purpose drilling system 230 has a primary borehole 232that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 236 that is drilled from the same location as theprimary borehole 232 from the surface 10 into the coal seam 12. As canbe seen, the secondary borehole 236 extends from the primary borehole232. The primary borehole 232 and the secondary borehole 236 extendtowards the install road 162.

The second multi purpose drilling system 330 has a primary borehole 332that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 336 that is drilled from the same location as theprimary borehole 332 from the surface 10 into the coal seam 12. Theprimary borehole 232 and the secondary borehole 236 extend towards theinstall road 162 from an area close to the ends of the primary borehole232 and the secondary borehole 236 of the first multi purpose drillingsystem 230.

Vertical wells 240, 340, 344 are drilled from the surface 10 so thatthey intersect with a corresponding primary borehole 232, 332 and acorresponding secondary borehole 236, 336. The vertical wells 240, 340,344 are spaced approximately 800-1000 meters from corresponding adjacentvertical wells 240, 340, 344. The vertical wells 240, 340, 344 conveycoal seam gasses to the surface 10.

FIG. 5 shows a facing configuration, where two multi purpose drillingsystems 430, 530 are drilled towards each other to degas a portion ofthe coal seam 12. The portion of the coal seam 12 to be degassed is aproposed longwall panel that extends from a main road 160 to an installroad 162. The arrow 164 indicates the direction of mining, from theinstall road 162 towards the main road 160.

The first multi purpose drilling system 430 has a primary borehole 432that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 436 that is drilled from the same location as theprimary borehole 432 from the surface 10 into the coal seam 12. Theprimary borehole 432 and the secondary borehole 436 extend towards theinstall road 162 and intersect a vertical well 544.

The second multi purpose drilling system 530 has a primary borehole 532that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 536 that is drilled from the same location as theprimary borehole 532 from the surface 10 into the coal seam 12. Theprimary borehole 532 and the secondary borehole 536 extend towards themain road 160 and intersect the vertical well 544.

Additional vertical wells 440, 540 are drilled from the surface 10 sothat they intersect with a corresponding primary borehole 432, 532 and acorresponding secondary borehole 436, 536. The vertical wells 440, 540,544 are spaced approximately 800-1000 meters from corresponding adjacentvertical wells 440, 540, 544. The vertical wells 440, 540, 544 conveycoal seam gasses to the surface 10.

FIG. 6 shows a combined configuration, where two multi purpose drillingsystems 630, 730 are drilled away from each other to degas a portion ofthe coal seam 12. The portion of the coal seam 12 to be degassed is aproposed longwall panel that extends from a main road 160 to an installroad 162. The arrow 164 indicates the direction of mining, from theinstall road 162 towards the main road 160.

The first multi purpose drilling system 630 has a primary borehole 632that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 636 that is drilled from the same location as theprimary borehole 632 from the surface 10 into the coal seam 12. Theprimary borehole 632 and the secondary borehole 636 extend towards themain road 160.

The second multi purpose drilling system 730 is drilled from the samelocation as the first multi purpose drilling system 630 and has aprimary borehole 732 that is drilled from the surface 10 to above thecoal seam 12 and a secondary borehole 736 that is drilled from the samelocation as the primary borehole 732 from the surface 10 into the coalseam 12. The primary borehole 732 and the secondary borehole 736 extendtowards the install road 162.

Vertical wells 640, 644, 740, 744 are drilled from the surface 10 sothat they intersect with a corresponding primary borehole 632, 732 and acorresponding secondary borehole 636, 736. The vertical wells 640, 644,740, 744 are spaced approximately 800-1000 meters from correspondingadjacent vertical wells 640, 644, 740, 744. The vertical wells 640, 644,740, 744 convey coal seam gasses to the surface 10.

FIG. 7 shows an ‘overlapping’ configuration (when viewed side on), wheretwo multi purpose drilling systems 830, 930 are drilled towards eachother and overlap to degas a portion of the coal seam 12. The portion ofthe coal seam 12 to be degassed is a proposed longwall panel thatextends from a main road 160 to an install road 162. The arrow 164indicates the direction of mining, from the install road 162 towards themain road 160.

The first multi purpose drilling system 830 has a primary borehole 832that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 836 that is drilled from the same location as theprimary borehole 832 from the surface 10 into the coal seam 12. Theprimary borehole 832 and the secondary borehole 836 extend towards theinstall road 162.

The second multi purpose drilling system 930 has a primary borehole 932that is drilled from the surface 10 to above the coal seam 12 and asecondary borehole 936 that is drilled from the same location as theprimary borehole 932 from the surface 10 into the coal seam 12. Theprimary borehole 932 and the secondary borehole 936 extend towards themain road 160.

Vertical wells 840, 844, 940, 944 are drilled from the surface 10 sothat they intersect with a corresponding primary borehole 832, 932 and acorresponding secondary borehole 836, 936. The vertical wells 840, 844,940, 944 are spaced approximately 800-1000 meters from correspondingadjacent vertical wells 840, 844, 940, 944. The vertical wells 840, 844,940, 944 convey coal seam gasses to the surface 10.

Although it is counterintuitive, as the present invention involves morecost and a greater amount of work upfront compared to known prior artsolutions, the applicant has found that the present invention can savecosts, production time and require less work in the long run for predrainage and drainage of coal seam gases during development, productionand after mining (goaf degassing).

The foregoing embodiments are illustrative only of the principles of theinvention, and various modifications and changes will readily occur tothose skilled in the art. The invention is capable of being practicedand carried out in various ways and in other embodiments. For example,it will be understood that the invention can be used for multi seam gasextraction. It is also to be understood that the terminology employedherein is for the purpose of description and should not be regarded aslimiting.

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

1. A method of forming a multi purpose drilling for degassing a coalseam that is to be mined, the method including the steps of: drilling aprimary borehole from a surface into a zone above the coal seam, suchthat at least a portion of the primary borehole extends substantiallyparallel to the coal seam; and drilling at least one secondary borehole;wherein the at least one secondary borehole extends from the primaryborehole into the coal seam.
 2. A method as claimed in claim 1, whereinthe step of drilling at least one secondary borehole involves drillingone secondary borehole into the coal seam such that a majority of thesecondary borehole in the coal seam extends substantially parallel tothe coal seam.
 3. A method as claimed in claim 2, further including thestep of drilling branching holes from the secondary borehole that extendinto the coal seam.
 4. A method as claimed in claim 1, wherein the stepof drilling at least one secondary borehole involves drilling aplurality of secondary boreholes such that they extend from the portionof the primary borehole that extends substantially parallel to the coalseam, and into the coal seam.
 5. A method as claimed in any one of thepreceding claims, wherein the step of drilling the primary boreholeinvolves drilling the primary borehole such that the majority of theprimary borehole in the zone above the coal seam extends substantiallyparallel to the coal seam.
 6. A method as claimed in any one of thepreceding claims, wherein a portion of the primary borehole is drilledsuch that it has a larger diameter than the average diameter of the atleast one secondary borehole.
 7. A method as claimed in any one of thepreceding claims, wherein the primary borehole is drilled such that theportion of the primary borehole extending substantially parallel to thecoal seam is larger in diameter than a portion of the primary boreholeadjacent to the surface.
 8. A method as claimed in any one of thepreceding claims, wherein the step of drilling the primary boreholeinvolves drilling a pilot hole and reaming at least a portion of thepilot hole.
 9. A method as claimed in any one of the preceding claims,wherein the step of drilling at least one secondary borehole involvesdrilling at least one secondary pilot hole and reaming each of the atleast one secondary pilot hole.
 10. A method as claimed in any one ofthe preceding claims, further including the step of drilling at leastone well from the surface, each of which intersects with the primaryborehole.
 11. A method as claimed in claim 10, wherein the step ofdrilling at least one well from the surface, each of which intersectswith the primary borehole, involves drilling the at least one well suchthat each of the at least one well also intersects with one of the atleast one secondary borehole.
 12. A method as claimed in claim 10 orclaim 11, wherein each of the at least one well is a vertical well. 13.A method as claimed in any one of the preceding claims, furtherincluding the step of providing a casing that is at least partiallyperforated in the portion of the primary borehole that extendssubstantially parallel to the coal seam.
 14. A method as claimed in anyone of the preceding claims, further including the step of providing acasing that is at least partially perforated in the at least onesecondary borehole.
 15. A multi purpose drilling system for degassing acoal seam that is to be mined, the multi purpose drilling systemcomprising: a primary borehole extending from a surface into a zoneabove the coal seam; and at least one secondary borehole extending fromthe primary borehole into the coal seam; wherein at least a portion ofthe primary borehole extends substantially parallel to the coal seam.16. A multi purpose drilling system as claimed in claim 15, wherein amajority of the secondary borehole in the coal seam extendssubstantially parallel to the coal seam.
 17. A multi purpose drillingsystem as claimed in claim 16, wherein branching holes extend from thesecondary borehole into the coal seam.
 18. A multi purpose drillingsystem as claimed in claim 15, wherein a plurality of secondaryboreholes extend into the coal seam from the portion of the primaryborehole that extends substantially parallel to the coal seam.
 19. Amulti purpose drilling system as claimed in any one of claims 15 to 18,wherein the majority of the primary borehole in the zone above the coalseam extends substantially parallel to the coal seam.
 20. A multipurpose drilling system as claimed in any one of claims 15 to 19,further comprising at least one well from the surface, each of whichintersects with the primary borehole.
 21. A multi purpose drillingsystem as claimed in any one of claims 15 to 20, further comprising acasing that is at least partially perforated in the portion of theprimary borehole that extends substantially parallel to the coal seam.22. A multi purpose drilling system as claimed in any one of claims 15to 21, further comprising a casing that is at least partially perforatedin the at least one secondary borehole.
 23. A multi purpose drillingsystem as claimed in any one of claims 15 to 22, wherein the portion ofthe primary borehole extending substantially parallel to the coal seamis larger in diameter than a portion of the primary borehole adjacent tothe surface.
 24. A method or multi purpose drilling system as claimed inany one of the preceding claims, wherein the zone is a primary cavingzone.
 25. A method or multi purpose drilling system as claimed in anyone of the preceding claims, wherein the zone is between 5 to 40 metersabove the coal seam.
 26. A method or multi purpose drilling system asclaimed in any one of the preceding claims, wherein the zone is between5 to 25 meters above the coal seam.
 27. A method or multi purposedrilling system as claimed in any one of the preceding claims, whereinthe zone is between 10 to 15 meters above the coal seam.